CMOS Image Sensor Deep P-Well Layout for Pixel Isolation
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Solution Overview
Problem
The challenge in CMOS image sensor fabrication is the high process complexity and cost associated with shrinking pixel size, leading to increased white pixels and pixel quality degradation due to the use of thick photoresist, as well as issues with blooming and crosstalk caused by discontinuous deep isolation trenches.
Innovation Solution
The solution involves a semiconductor substrate design with deep isolation trenches and P-well blocks that are separated at their crossing points, allowing for reduced process costs and improved pixel quality by using a single photolithography process and P-type ion implantation to form the first deep P-well blocks, which provide electrical isolation and repair damage from the dry etching process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If deep P-well formation with high dose ion implantation is used to isolate photodiodes, then pixel isolation and prevention of blooming/crosstalk is improved, but process complexity and cost increase due to requiring thick photoresist (>2 μm)
Solution Approach 1:
The patent segments the isolation structure into two distinct components: deep isolation trenches (providing physical separation) and deep P-well blocks (providing electrical isolation). This segmentation allows each component to perform its specialized function, eliminating the need for thick photoresist while achieving both physical and electrical isolation of photodiodes, thereby reducing process complexity without compromising isolation quality
Solution Approach 2:
The patent employs a composite isolation architecture combining dielectric material-filled trenches with doped semiconductor regions (deep P-well blocks). This composite approach integrates physical barriers (trenches) with electrical barriers (doped regions), achieving superior isolation效果和 preventing blooming/crosstalk without requiring thick photoresist, thus resolving the contradiction between isolation effectiveness and process simplicity
2Manufacturing precision
If thick photoresist is used for deep P-well formation, then adequate ion implantation depth is achieved, but white pixels increase and pixel quality deteriorates
Solution Approach 1:
The patent extracts the ion implantation step from the thick photoresist process by forming deep P-well blocks through selective doping in predefined regions. This separates the function of defining isolation regions (done through trench patterning) from the function of creating doped regions (done through targeted ion implantation), allowing precise depth control without relying on thick photoresist, thereby eliminating white pixel defects while maintaining manufacturing precision
Solution Approach 2:
The patent performs preliminary patterning of deep isolation trenches before forming deep P-well blocks. This preliminary action defines the exact locations where ion implantation will occur, ensuring precise depth and location control of P-well blocks without requiring thick photoresist. The pre-defined trench structures serve as templates for subsequent doping, eliminating the white pixel problem associated with thick photoresist while maintaining precise implantation depth control
3Area of moving object
If pixel-to-pixel pitch is reduced to arrange more pixels in same area, then pixel density and image definition are improved, but process complexity increases exponentially due to high aspect ratio photoresist requirements
Solution Approach 1:
The patent segments the isolation function into physical trenches and electrical P-well blocks, allowing the use of thinner photoresist that can accommodate smaller pixel pitches without exponential increase in process complexity. The segmented approach enables independent optimization of trench dimensions and P-well block dimensions, making the process scalable to higher pixel densities without the exponential complexity penalty
Solution Approach 2:
The patent changes the critical parameter from photoresist thickness to trench depth and P-well block depth. This parameter change allows the use of thinner, more manageable photoresist layers that can be processed with standard lithography tools even at reduced pixel pitches, eliminating the exponential complexity increase that would otherwise result from requiring ultra-thick photoresist for high aspect ratio structures
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design reduces white pixels, enhances pixel quality, and addresses blooming and crosstalk issues by ensuring electrical and optical isolation between photodiodes, resulting in improved image sensor performance with reduced process complexity and cost.
Implementation Method 1
Each photosensitive element absorbs a part of the incident image light and immediately generates an image charge upon the absorption
Implementation Method 2
the formation of such a layer involving the implantation of a high dose of ions
Data Source
AI summary
The present invention provides an image sensor and a method for fabricating the image sensor. The image sensor includes a semiconductor substrate, photodiodes (PD), deep isolation trenches, first deep P-well blocks and second deep P-wells. The isolation trenches surrounds the PDs, and each of the isolation trenches is separated from any adjacent isolation trenches at a crossing where they would have crossed each other if they further extended. The first deep P-wells blocks have projections on the semiconductor substrate, which encompass projections of said separated portions of the isolation trenches on the semiconductor substrate. The first P-well blocks can be formed using only one photomask, resulting in a reduction in cost.